Altitude affects different people in different ways, but common symptoms of altitude sickness include headache, nausea, loss of appetite, trouble with sleep, and lack of energy.  To help prevent altitude sickness, we recommend you always remain well hydrated, gradually expose yourself to higher elevations, exercise moderately until altitude acclimatized, and get plenty of rest.  Most importantly, we suggest that you arrive in Cusco at least 1 to 2 days prior to the first day of your trip (we include acclimatization days on  our Classic Inca Trail Trek). Because altitude issues are intricately tied to dehydration, you should increase your fluid intake and limit alcoholic beverages, caffeine, and aspirin, which tend to dehydrate your system.